A radio access network allows entities to communicate with each other over a wireless, radio channel. For example, a user equipment (UE) is able to communicate with network elements such as a base station (or “Node B”) over a radio access network. When entities communicate with each other they must conform to standard protocols so that each entity can correctly interpret the data received from other entities. Third Generation Partnership Project (3GPP) is one wireless cellular standard protocol which can be used.
Hybrid Automatic Repeat Request (HARQ) communication may be implemented within the 3GPP protocol. In HARQ communication, data blocks are coded for transmission from a transmitter (e.g. a Node B) to a receiver (e.g. a UE). As coded data blocks are transmitted from the transmitter to the receiver, some of the data blocks may become corrupted or lost during transmission. This may cause a problem for the receiver when the receiver attempts to decode the data blocks. According to the HARQ protocol data blocks may be coded to include error detection bits (e.g. cyclic redundancy check (CRC) bits) and/or error correction bits (e.g. forward error correction (FEC) bits) to help the receiver to correctly decode the data blocks.
A receiver will attempt to decode coded data blocks received from a transmitter, and may use error correction bits included with the data blocks for this decoding. The error detection bits are used to determine whether the data blocks have been correctly decoded at the receiver, e.g. by performing a CRC check on the decoded data blocks. Failure of decoding a data block is indicated by a failure of the CRC check for the constituent data bits. Data blocks which are not decoded successfully by the receiver on the first transmission attempt may be periodically re-transmitted by the transmitter until they are successfully decoded by the receiver. In some systems, the receiver may request retransmission of incorrectly decoded data blocks. In other systems the transmitter may periodically transmit the coded data blocks until an acknowledgement is received from the receiver to indicate that the data blocks have been correctly decoded at the receiver.
In a receiver implementing the 3GPP Long Term Evolution (LTE) protocol, in order to decode the received coded data bits of a data block, the receiver determines soft information values (e.g. Log-Likelihood Ratio (LLR) values) for the respective coded bits, where each soft information value is 8 bits and indicates the likelihood of a respective coded data bit being a zero. In the case where the soft information values (or “HARQ information”) comprise 8-bit LLR values that are signed, each LLR value gives a value in the range [−127, 127]. A positive value indicates a higher chance of the corresponding coded bit (or “codeword bit”) being 0, while a negative value indicates a higher chance of the coded bit being 1. For example, an LLR value of:                −127 indicates that the corresponding coded bit is “definitely 1”;        −100 indicates that the corresponding coded bit is “very likely 1”;        0 indicates that the corresponding coded bit “could be either 0 or 1 with equal probability”;        100 indicates that the corresponding coded bit is “very likely 0”; and        127 indicates that the corresponding coded bit is “definitely 0”.        
In HARQ communication, the soft information values (e.g. LLR values for each turbo-coded transmitted bit of the data block) which are used in each failed decoding attempt are stored and re-used for future decoding attempts. When a coded data block is retransmitted, the receiver determines the soft information values for the retransmitted coded data bits of the coded data block and combines these new soft information values with the soft information values stored from previous attempts to decode the data block. The combined soft information values can then be used to attempt to decode the coded data block.